| Basis for focus area |
Data-driven prospectivity ranking of watersheds (HUCs; Karl and others, 2016) shows that the focus area includes 35,060 km2 with a total of 65 HUCs ranked as highly prospective. The belt is a regional geochemical anomaly with a number of mid to Early Cretaceous alkaline and peralkaline syenite, lamprophyre, monzonite, bostonite, and granite intrusive complexes, and known granite-related U. Includes ARDF occurrences of REE, uranium, thorium, tungsten, and copper at Kwinniuk River, Golovin Bay, and Clear Creek. |
| Identified resources |
None. |
| Production |
None recorded. |
| Status |
Unknown. |
| Estimated resources |
Unknown. |
| Geologic maps |
Less than 10% of the area has been mapped at 1:63,360 scale or better. |
| Geophysical data |
Inadequate aeromagnetic and aeroradiometric coverage. |
| Favorable rocks and structures |
Kachauik monzonite and syenite pluton phases are intruded by a series of nepheline-syenite dikes with highly anomalous REE and uranium-thorium. Extending east of the Kachauik pluton are a series of alkaline and subalkaline monzonite to syenite intrusions of monzonite to syenite composition. Total lithology scores of the high prospectivity HUCs ranges from 1-1 (out of a maximum possible of 8) resulting in an average of 1, whereas igneous scores range from 0-3 with an average of 1.14. Alkaline and calc-alkaline intrusive rocks in the Darby Mountains, including nepheline-syenite, high-K dike swarms are known to contain REE-bearing minerals such as allanite, vesuvianite, and melanite garnets. |
| Deposits |
None. |
| Evidence from mineral occurrences |
Known ARDF occurrences associated with granite uranium-REE deposits and associated placers occur on the eastern extent of the belt. Multiple notable occurrences are scattered across the Darby Mountains, both in hard rock occurrences and placers; ARDF: SO038; SO003; SO036; SO037; SO039; SO040; SO041; SO042; SO043; SO044; SO159; ARDF scores range from 0-2 with an average of 0.55. |
| Geochemical evidence |
Sediment geochemistry scores range from 3-6 with an average of 4.98. Anomalous sediment geochemistry and heavy mineral concentrates are reported by Barker (1991a, b, c) and others across the Darby Mountains. |
| Geophysical evidence |
Aeroradiometric scores range from 0-1 with an average of 0.66. |
| Evidence from other sources |
No data. |
| Comments |
This belt contains known REE-bearing mineral occurrences associated with alkaline intrusive complexes, and uranium-thorium mineralization. Regionally, the belt is a geochemical anomaly exhibiting LREE and pathfinder anomalies. Geologically, the belt is favorable for REE mineralization of unknown potential. Recent field work by USGS in the Darby Mountains shows a strong correlation between alkaline igneous rocks and REE enrichments. Allanite is widespread throughout the intrusions, and is generally concentrated in the alkaline, high-K syenites and dike swarms. These occurrences are in rocks that have similar compositions to those spread across about 500 km from Saint Lawrence Island through the Zane Hills. |
| Cover thickness and description |
Unknown. |
| Authors |
Douglas C. Kreiner, James V. Jones III, Melanie B. Werdon. |
| New data needs |
New mapping, geophysics, updated geochemical sampling. |
| Geologic mapping and modeling needs |
Framework geology, including mapping of lithology, structure, and hydrothermal alteration. Detailed mapping of alkaline intrusive phases is recommended, paying particular attention to hydrothermal assemblages, compositions and orientations of dikes, and providing updated geochemical and geochronologic data of igneous events and their respective petrogenesis. Host rock mapping and characterization will help understand possible crustal enrichments. Structural observations should focus on evidence of tectonic environment of emplacement, and possible fluid pathways. Isotopic data (Hf, Pb) will test crustal character and terrane affinity. Systematic, detailed geologic mapping should be conducted for areas containing and surrounding ARDF records and potentially correlative igneous units throughout the region. |
| Geophysical survey and modeling needs |
Rank 1 or 2 aeromagnetic data over the entire belt. Targeted EM where relevant for lithologic and structural mapping under cover. |
| Digital elevation data needs |
IfSAR coverage is complete over entire focus area. |
